The invention relates to apparatus for segregating solid particles and other impurities from a gaseous carrier medium, and more particularly to improvements in so-called electrostatic precipitators.
An electrostatic precipitator comprises a housing wherein a stream of gaseous carrier medium which contains solid particles and/or other impurities is conveyed past one or more electrostatic filters each of which can comprise a thin wire-like corona discharge electrode surrounded by a tubular collecting electrode. Solid particles and other impurities deposit on the collecting electrode and are evacuated by way of one or more collecting vessels at the bottom of the housing. The housing has an inlet for admission of untreated carrier medium and an outlet for evacuation of treated carrier medium. In many instances, the outlet includes a funnel-shaped section which tapers in the direction of flow of treated gaseous carrier medium therein and discharges such medium into a conduit. The just described electrostatic precipitators are often used for removal of impurities from industrial gases (e.g., flue gases in steam plants), for removal of acid mists in chemical process plants, for cleaning of air in ventilation and air conditioning systems, and for many other purposes.
The once treated gaseous carrier medium normally still contains a certain percentage of solid particles and/or other impurities (hereinafter referred to as solid particles). The carrier medium normally includes an upper layer which is devoid of solid particles or contains only a very small percentage of solid particles, and a lower layer which contains a higher percentage of solids. Such stratification of treated gaseous carrier medium is attributable to the nature of flow and/or velocity of carrier medium in the electrostatic precipitator.
In accordance with a proposal which is disclosed in European Pat. No. 0162826 Al, the outlet of the electrostatic precipitator is provided with an opening which allows for evacuation of some of the treated carrier medium along a separate path. The thus withdrawn portion of the carrier medium is then treated in a separate precipitator prior to being admitted into the conduit for the remaining part of the treated carrier medium. Alternatively, the segregated portion of the treated carrier medium is subjected to a secondary purifying treatment and is then introduced into the inlet of the electrostatic precipitator for renewed (third) treatment. The segregated portion constitutes a very small percentage of the once treated carrier medium. It has been found that such mode of diverting some of the treated medium does not ensure reliable segregation of a large percentage of solid particles from the once-treated gaseous carrier medium.
German Pat. No. 314,171 proposes to provide the outlet of the precipitator with lateral channels which divert some of the treated gaseous carrier medium. Such proposal is unsatisfactory because the highest percentage of solid particles can be found in the lower layer of the once treated carrier medium.
German Offenlegungsschrift No. 1457095 proposes to divide the conduit for once treated carrier medium into several smaller conduits which receive carrier medium by way of a funnel-shaped duct tapering in a direction from the purifying station toward the intake ends of the smaller conduits. The bottom wall of the funnel-shaped section slopes upwardly toward the intake ends of the smaller conduits. Solid particles which travel along such upwardly sloping wall enter a turbulent zone of the carrier medium at the intake ends of the conduits so that the solid particles are mixed with the carrier medium and their distribution in the conduits is unpredictable. Consequently, the uppermost conduit or two or more upper conduits are just as likely to contain a relatively high percentage of solid particles as the lowermost or lower conduits so that the apparatus of this publication is incapable of ensuring predictable subdivision of the outflowing stream of treated carrier medium into several smaller streams containing different quantities of solid particles.